Electroplating 3D printing is one of the most important electrodeposition-based additive manufacturing technologies. Compared with traditional manufacturing technology, it has some distinct advantages such as high dimensional accuracy and great process repeatability. However, the thickness non-uniformity of the electrodeposited coatings is a bottleneck problem for the further development of the technology. This problem seriously affects the accuracy, availability, and mechanical properties of the printed parts. The work aims to optimize the electroplating 3D printing process and improve the thickness uniformity of single-layer electrodeposition. The multi-physical field theory model coupling electrolyte flow, mass transfer, and electrode reaction was established and numerically solved. The evolution law of the distribution cloud diagram of reactive ion concentration and electrolyte potential as well as deposited layer morphology with time was studied. Combined with electrode reaction kinetics, the direct factors affecting the thickness uniformity of the deposited layer were analyzed. Finally, the effect of main factors on the thickness uniformity of the deposited layer was discussed, such as inhibitor concentration in bulk solution CS, fluid velocity in bulk solution u0, conductivity of the electrolyte κ, and cathode potential φc. The simulation results indicated that with the increase of deposition time, the morphology of the deposited layer became more and more uneven. The distribution cloud diagram of reactive ion concentration and electrolyte potential varied with the morphology of the deposited layer. The distribution uniformity of reactive ion concentration and overpotential on the surface of the deposited layer was improved after addition of 40 μmol/L inhibitor. Generally, as the inhibitor concentration in bulk solution increased, the morphology of the deposited layer became smoother and the thickness uniformity of the deposited layer became better. However, there was a saturated concentration of 40 μmol/L. When the saturation concentration exceeded the saturated concentration, the thickness uniformity of the deposited layer did not increase with the increase of the concentration of inhibitor. The morphology of the deposited layer was usually saddle-shaped. The thickness of the left side of the deposited layer was slightly higher than that of the right side at a low fluid velocity. As the fluid velocity increased, the morphology of the deposited layer gradually changed to a saddle shape with a lower left and a higher right. At low conductivity, the deposited layer exhibited a saddle-shaped morphology. With the increase of conductivity, the morphology of the deposited layer gradually changed to a hump-shaped morphology. The lower the cathode potential was, the more obvious the saddle shape features were. The thickness uniformity of the deposited layer firstly increased and then decreased with the increase of fluid velocity or electrolyte conductivity, and monotonically increased with the increase of cathode potential. The non-uniform distribution of reactive ion concentration and overpotential on the cathode surface directly leads to the non-uniformity of deposited layer thickness. At the same time, the changing morphology of the deposited layer can react on the distribution of reactive ion concentration and overpotential on the cathode surface. Adding enough inhibitors can effectively improve the thickness uniformity of the deposited layer. Too large or too small fluid velocity or conductivity will reduce the thickness uniformity of the deposited layer. The thickness uniformity of the deposited layer can be improved by appropriately increasing the cathode potential.